1. Field of the Invention
This invention relates to electrostatic powder spraying, and more particularly to spraying dry blended powders, such as those containing mica or metallic particles, or spraying other powders containing electrically conductive material.
2. Description of the Prior Art
Various electrostatic spray coating processes and guns used for charging particles of a coating material emitted from the gun are well known. In a typical electrostatic powder spraying process, powder is conveyed in an air stream to a spray gun. The powder is electrically charged to some high voltage potential. The charging system generally operates at a potential of at least 60 kilovolts (KV). In the usual situation, electrostatic powder spray systems are used to coat electrically conductive or at least partially conductive objects. To attract the coating material to the object to be coated, the coating particles are charged to a different polarity than the object to be coated. The objects to be coated are usually held at ground voltage potential by some appropriate means. Electrostatic forces between the charged powder and the grounded conductive object cause the powder to be drawn to the object to be coated.
In order to charge the powder in an electrostatic spray coating system, it is necessary to have some means of applying the charge to the powder. Charging is usually accomplished in the spray gun by an electrode connected to a high voltage power supply. The electrode is placed in close proximity to or even in contact with the stream of powder.
Early forms of electrostatic spray guns were powered from remote high-voltage d.c. supplies that provided output voltages of 60 KV or higher. The output voltage of such power supplies was conducted via high voltage cables to particle-charging electrodes mounted near the nozzles of the guns. To provide a safe and more maneuverable gun, miniaturized voltage multiplier circuits operating at high frequency were developed that could fit within the electrostatic spray gun to produce the requisite high d.c. charging voltage from a lower input voltage. Such guns with internal high voltage multiplication capabilities are generally powered from an external low voltage power supply via a low voltage cable to the gun which is more flexible than high voltage cables. The integral high voltage circuit steps up the low input voltage by means of a transformer, rectifies and multiplies the step-up voltage in a diode/capacitor multiplier cascade, and outputs a high d.c. voltage to the particle-charging electrode of the gun.
U.S. Pat. No. 5,026,720 discloses an electrostatic spray gun construction that can be powered from an internal low voltage multiplier circuit fed from an external lower voltage source.
These apparatus and processes work well when spraying nonconductive powders and powders made of a single material. However, several problems occur with electrostatic powder spray guns when spraying dry blended powders, such as those containing mica or metallic particles. Due to the different molecular surface structure of the individual powder particles, electrostatic separation occurs. Some of these different particles accumulate on the inside of the powder flow path, attracting other like particles, and continue to build until their electrostatic charge can no longer keep them attached to the inside of the powder path. At that time, they can break off as a soft lump of powder, usually different in color, and cause an imperfection on the surface of the object being coated.
In order to prevent the undesired build-up of powder along the inside of the powder flow path, and the associated break up of accumulations of powder, the spray gun should be cleaned at frequent intervals. If the gun is not cleaned every few minutes under continuous operation, these accumulations will tend to occur. However, cleaning the gun can be a relatively time-consuming and inefficient procedure, especially if the gun must be disassembled or partially disassembled in order to clean the inside of the powder flow path.
Another problem is caused when spraying powder containing metallic particles. When spraying powders which are moderately conductive, such as metallic powders, certain precautions must be taken to prevent the high voltage at the electrode from being short circuited to ground through the column of powder being supplied to the gun. When using an electrostatic field generated by high voltage, such as 100 KV, metal particles suspended in a powder stream are close enough to each other to form a conductive chain. The voltage travels down the powder supply path looking for a neutralizing source. Unfortunately, when using hand-held powder spray guns, a neutralizing source or ground path may travel through the operator's hand, and the discharge of voltage can be irritating and somewhat painful to the operator. The formation of this facilitated grounding path by the powder can also result in a high current draw into the gun, and this high current draw can result in poor transfer efficiency.
Similar grounding problems have occurred when spraying liquid paint which is moderately conductive in electrostatic spray guns. One approach has been to isolate the entire supply from ground potential. This would allow the entire spray system to "float" at the charging potential. However, such an approach has several drawbacks. One of the major drawbacks is that an enormous amount of electrical energy would be capacitively stored in the system. This capacitively stored energy could inadvertently be discharged in a spark, causing either an electrical shock to operating personnel, or possibly causing an explosion.
Another approach has been to ground the supply container, and to connect the spray gun to the container with a hose which is long enough to make the total electrical resistance of the material column between the gun and the container large enough for moderately conductive materials, so as to reduce electrical current through the material column to a level that would not short out the electrode. However, this approach has distinct disadvantages. The supply hose in such systems would be necessarily very bulky and hard to manage when used with a hand-held gun. These hoses would have to be bulky in order to provide the necessary electrical insulation, and possibly would even include a grounded conductive layer surrounding the hose. From the operator's point of view, this approach would be very burdensome.
Another solution for isolating the circuit in liquid spray guns is shown in U.S. Pat. No. 4,139,155, in which a fluid cartridge is installed at the connection of the hose to the gun. The cartridge has an elongated spiral passage in place of the usual straight passage, presenting an increased resistive grounding path, and effectively isolating the gun from ground. While the approach of U.S. Pat. No. 4,139,155 works for liquid, it is not suitable for use with powder spray guns, since the air conveyance used with powder flow cannot be pressurized in the same way that liquid flow is. The convoluted path provided by the elongated spiral passage would provide undesirable flow resistance to the flow of the airstream containing the powder and decrease the velocity of the powder reaching the gun. As a result powder conveyance and pumping equipment currently being used would no longer be suitable, and new equipment would need to be provided.
Another solution would be to provide a ground path for the powder at the connection of the powder supply hose to the gun. Such a ground path would provide a ground path of less resistance parallel to the powder in the supply hose. However, the provision of such a low resistance grounding path could result in a high current draw into the gun, which, in effective, tends to short out the electrode. This high current draw can result in poor transfer efficiency since it makes it more difficult to impart the proper charge on the powder passing by the electrode.